1. Field of the Invention
The present invention relates generally to a receiving apparatus and method of a mobile station in a high rate packet data system using a Code Division Multiple Access (CDMA) scheme, and in particular, to an apparatus and method for controlling a dynamic range of weight vectors according to various combining methods to fit an input range of a channel decoder in a mobile station using the various combining methods.
2. Description of the Related Art
In general, mobile communication systems are classified based on communication methods into a Frequency Division Multiple Access (FDMA) scheme in which a predetermined frequency band is divided into a plurality of frequency channels, each channel being assigned to an individual subscriber a Time Division Multiple Access (TDMA) scheme in which a plurality of subscribers share a single frequency channel by time-slicing the frequency channel, and a Code Division Multiple Access (CDMA) scheme in which a plurality of subscribers share the same frequency band at the same time by using different codes assigned thereto. The rapid development of communication technology has allowed mobile communication systems to provide a conventional phone call service and a high-speed data service for transmitting not only email or still images but also bulk digital data, such as a moving picture, in a mobile station.
Representative examples of mobile communication systems providing the high-speed data service using the CDMA scheme are Evolution Data Only (EV-DO) in which data can be transmitted at a high rate and Evolution of Data and Voice (EV-DV) suggested to cover a problem of EV-DO that a voice service and a data service cannot be supported at the same time.
In high rate packet data systems using the CDMA scheme, it is difficult to provide a high-speed packet data service of good quality due to frequency selective fading according to multiple paths, which occurs when an RF signal transmitted from a base station arrives at a mobile station after being reflected by various scatters near the mobile station, time selective fading according to Doppler spread, which occurs when a mobile station moves, and co-channel interference influenced by an adjacent base station when frequency reusability is close to 1.
In order to address this problem, Multiple Input Multiple Output (MIMO) technology, which is one of multiple antenna schemes, has been suggested. When a base station transmits high-speed packet data to a mobile station, if the number of antennas of the base station, which is related to transmit diversity, is increased, and if the number of antennas of the mobile station, which is related to receive diversity, is increased, a diversity gain is increased. Using these schemes, the fading and interference described above can be effectively reduced, angle spread of paths received to each antenna of the mobile station can be reduced, and if an incident angle of each path is different, and interference due to multiple paths can be reduced using multiple antennas, and accordingly, communication systems using multiple antennas has been developed.
However, since the size of each of mobile stations is limited, it is difficult in reality to equip the mobile station with more than two antennas in. When a mobile station moves from a base station, with which the mobile station is maintaining a current link, to an adjacent base station, the action that the mobile station connects a link to the adjacent base station while maintaining the current link is called soft handoff. Since each base station in a synchronous CDMA system has a unique short Pseudo-Noise (PN) code, a mobile station can maintain links with two or three base stations at the same time while maintaining a frequency reusability of 1.
However, in a TDMA system, a mobile station cannot communicate with two base stations in the same time slot at the same time, and in and FDMA system, a mobile station cannot communicate with two base stations in the same frequency slot at the same time, and thus, the implementation of soft handoff with frequency reusability close to 1 in the TDMA or FDMA scheme is more difficult than in the CDMA scheme. However, when the CDMA scheme is used, soft handoff is easy to implement, but in a case where a mobile station maintains a link with a single base station without soft handoff, a signal received from an adjacent base station acts as interference.
In the CDMA scheme, Walsh code having orthogonality unique to every mobile station is used. The orthogonality is satisfied only if Walsh codes are arranged at the same time point in a time axis. Walsh codes arranged at different time points act as significant interference in a multi-path environment due to non-orthogonality. Short PN code lessens the interference due to non-orthogonality. That is, since two short PN codes apart more than one chip from each other have an autocorrelation coefficient close to 0, which is inverse proportional to a code length, energy of a signal, which will act as interference, among signals passing through a Walsh decoder is reduced. Nonetheless, if the intensity of a signal received from an adjacent base station, which acts as an interference, is large, since the intensity of a signal received from a serving base station maintaining a current link is relatively low, the serving base station must radiate an electronic wave by properly performing power control.
In this case, the power control is necessary, and unlike a voice service for which an electronic wave is transmitted to a plurality of mobile stations at the same time, base stations in an EV-DO or EV-DV system transmit data to a single mobile station at the same time. Thus, in an EV-DO or EV-DV system, a forward packet data channel shows a time division characteristic, and a data rate is determined by means of rate control instead of power control. For a high-speed data service, a forward packet data channel uses a plurality of Walsh codes, and if soft handoff is supported using all Walsh codes, it is complicated for two or three base stations to assign a time slot in the time axis for a single mobile station.
Thus, EV-DO and EV-DV systems suggested by the Third Generation Partnership Project 2 (3GPP2), which is a standardization organization, are defined not to use soft handoff in a forward packet data channel. Accordingly, since packet data transmitted from an adjacent base station to another mobile station acts as an interference signal in the same channel to packet data received by a specific mobile station from a base station maintaining a current link, a technique for reducing the interference signal is required.
In order to reduce the interference signal, a receive end of a mobile station equipped with multiple antennas in an EV-DO or EV-DV system includes a RAKE receiver, assigning a finger to each of multiple paths, descrambling short PN code signals, multiplying the descrambled signals by proper weight vectors by performing channel estimation, and combining the multiplied signals. The combined signal is despread by a Walsh decoder, and transmitted bits are determined by a soft metric generator and an error correction decoder.
The combined signal obtained by multiplying the descrambled signals by weight vectors is input to a channel decoder, and various combining methods for calculating the weight vectors exist. Although a range of the combined signal varies according to each of the combining methods, an input range of the channel decoder to which the combined signal is input is limited in general. Thus, a technique for controlling a dynamic range of the weight vectors to fit the input range of the channel decoder even using the various combining method is required.